The invention relates generally to the field of electric motors and specifically to an AC motor with improved performance characteristics.
Many types of electric motors are known to the industry. Typically, these known motors have certain desirable characteristics such as high starting torque, variable speed and/or high power density. Often, however, a motor with desirable characteristics for a given application has certain disadvantages or deficiencies. These undesirable characteristics often include relatively high cost, electrical circuit complexity, radio frequency or electromagnetic interference, energy inefficiency, limited reliability and/or comparatively short service life.
The invention provides an AC power operated electric motor that exhibits desirable torque/speed characteristics when operated in an open loop condition and is effectively speed and/or torque controlled with relatively simple and economical electrical circuitry. The motor has a stator with field windings that are energized with alternating current and that are arranged to induce an AC current in a conductive loop on a rotor or armature. In various configurations of the motor, the field windings comprise at least two coils angularly displaced from one another around the rotor axis. The positions of the windings in some configurations represent physically or mechanically distinct phases.
The AC stator field is caused to move about the axis of the rotor and the induced AC field in the conductive loop produces a torque on the rotor causing it to rotate in synchronization with the field rotation. The rotation of the stator field is produced by switching or appropriately modulating AC power to successive angularly displaced field coils.
The motor can be arranged with 2, 4, 6 or even a greater number of even poles and with as many field winding phases as suitable for a particular application. Motor torque, and therefore power, is multiplied in proportion to the number of poles provided in the motor. The motor has open loop speed/torque characteristics approaching the desirable ideal of constant horsepower. These characteristics include high starting torque and high speed at low load.
Importantly, the motor lends itself to relatively simple and energy efficient speed control and/or torque control. A standard speed control over a 10:1 ratio is readily achieved. Rated torque can be achieved at zero speed with proper circuitry and therefore the speed range can be from zero to the maximum rated speed. Some of the additional advantages of the motor include low stall current, operation on simple square wave power without difficulty with harmonics, and increased power and/or torque for a given physical size motor as compared to conventional induction motors, for example.